A gas turbine generally includes a compressor section, a combustion section having a combustor, and a turbine section. The compressor section progressively increases the pressure of the working fluid to supply a compressed working fluid to the combustion section. The compressed working fluid is routed through a fuel nozzle that extends axially within a forward, or head, end of the combustor. A fuel is combined with the flow of the compressed working fluid to form a combustible mixture. The combustible mixture is burned within a combustion chamber to generate combustion gases having a high temperature, pressure and velocity. The combustion chamber is defined by one or more liners or ducts that define a hot gas path through which the combustion gases are conveyed into the turbine section. In a can-annular type combustion system, multiple combustion cans (each having its own fuel nozzle(s) and liner) produce combustion gases that drive the turbine section.
The combustion gases expand as they flow through the turbine section to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity. The turbine may also drive the compressor by means of a common shaft or rotor.
In the combustor section, the fuel nozzles may operate solely on gaseous fuel, solely on liquid fuel, or simultaneously on gaseous fuel and liquid fuel. In many instances, a power-generation plant may experience occasions when it is necessary to operate for a given time using only liquid fuel. In these instances, plant operators have found it convenient to transition from gaseous fuel operation to liquid fuel operation. However, occasions arise during which the primary gaseous fuel supply is unavailable. During these occasions, it has been a challenge to ignite the liquid fuel at start-up without relying on the primary gaseous fuel supply.
One challenge with the ignition of liquid fuel at start-up lies in ensuring the proximity of the igniter to a region of ignitable liquid spray. If the igniter is not sufficiently close to the ignitable liquid spray, ignition will fail to occur. Some legacy ignition systems have relied on a spark igniter positioned within the flame zone and then retracted due to the pressure of the ignited combustion gases. Such spark igniters may experience accelerated wear due to their proximity to the hot combustion gases, particularly if the retraction mechanism fails to perform properly.
A related challenge with the ignition of liquid fuel is the regular production of a high-quality spray of the liquid fuel. Ideally, the liquid fuel spray has a uniformly fine droplet size, and the droplets are spread over a wide area of the combustion zone without reaching the liner walls. In less-than-ideal conditions, the spark igniter may be insufficient for reliable liquid fuel ignition, in instances when the atomized liquid fuel spray may be irregular or may be inadequate to reach the location of the igniter.
Finally, a third challenge with the ignition of liquid fuel occurs in those combustion systems that use cross-fire tubes to propagate a flame among an array of combustion cans. In these systems, if the flammable gases fail to span the width of the combustion can (and thereby enter the cross-fire tubes), proper cross-firing of the combustion cans will fail to occur. This problem may be exacerbated when the liquid fuel is delivered from a centrally located liquid fuel cartridge.
Therefore, an improved system for reliably igniting a liquid fuel in a combustion chamber would be useful, particularly in those circumstances where the power-generating plant may be experiencing a depletion or outage of the primary natural gas supply.